Characterization of Degradation Mechanisms in Neural Recording Electrodes
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1009-U05-08
Characterization of Degradation Mechanisms in Neural Recording Electrodes* Jennifer M. Anton, and Stephanie A. Hooker Materials Reliability Division, National Institute of Standards and Technology, 325 Broadway, MS 853, Boulder, CO, 80305 Understanding the nature of specific neural activity is essential to the progression of research in the field of brain disorders and diseases, as well as neuroprosthetics. Microelectrodes are the primary measurement devices used to transduce neural activity into electrical signals to help neuroscientists study dynamic brain function. Advances in signal processing and packaging currently allow neural recording for periods as long as a year by use of chronically implanted electrodes in freely behaving animals. Stability of the electrode impedance is required for optimum signal recording over the length of the recording interval. While electrode–tissue interaction plays a major role in the quality of the recorded signal, delamination or degradation of the dielectric coating also interferes with signal recording. Therefore, to improve the signal recording process, it is useful to understand how the electrode design and component materials affect the signal over the course of time. Commercially available platinum iridium microelectrodes coated with parylene-C were studied to assess the coating response to the test solutions described in Table I. HCl and H2O2 were chosen to simulate a chemical environment that could be present in vivo during signal measurement. Electrical characteristics of eight microelectrodes were evaluated by electrochemical impedance spectroscopy (EIS), where changes in the coating and metal surface corresponded to changes in the impedance spectrum. Electrodes of varying nominal impedance were immersed in test solutions and measured periodically over 26 days. (Nominal impedance was specified by the manufacturer as ± 20 % at 1 kHz.) A three electrode cell was used with a Ag/AgCl reference electrode. A platinum wire served as the counter electrode, and the working electrode was the microelectrode under investigation. The applied AC signal was 10 mV over a frequency range of 100 mHz to 100 kHz. Each electrode was inspected by SEM to visually assess the physical condition of the coating and electrode surface after electrochemical testing. Table I. Test solutions in which the microelectrodes were immersed. Test solutions based on Ringer’s solution (an aqueous solution of NaCl, KCl, and CaCl2)
Test solutions based on phosphate buffered saline, 1X (PBS)
Ringer’s solution
PBS
Ringer’s solution + HCl (0.18 %)
PBS + HCl (0.18 %)
Ringer’s solution + H2O2 (0.71 %)
PBS + H2O2 (0.71 %)
*Contribution of the U.S. Department of Commerce; not subject to copyright in the U.S.A.
An untested electrode was imaged to obtain a baseline condition for comparison against tested electrodes, shown in figure 1. The parylene coating is relatively smooth and the interface with the metal is intact. At the conclusion of the test period, all electrodes showed some degree
Pt-Ir
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